Variation in the access to deep soil water pools explains tree-to-tree differences in drought-triggered dieback of Mediterranean oaks

Individual differences in the access to deep soil water pools may explain the differential damage among coexisting, conspecific trees as a consequence of drought-induced dieback. We addressed this issue by comparing the responses to a severe drought of three Mediterranean oak species with different drought tolerance, Quercus pubescens L. and Quercus frainetto Ten., mainly thriving at xeric and mesic sites, respectively, and Quercus cerris L., which dominates at intermediate sites. For each species, we compared coexisting declining (D) and non-declining (ND) trees. The stable isotope composition (δ2H, δ18O) of xylem and soil water was used to infer a differential use of soil water sources. We also measured tree size and radial growth to quantify the long-term divergence of wood production between D and ND trees and non-structural carbohydrates (NSCs) in sapwood to evaluate if D trees presented lower NSC values. The ND trees had access to deeper soil water than D trees except in Q. frainetto, as indicated by significantly more depleted xylem water values. However, a strong δ2H offset between soil and xylem water isotopes observed in peak summer could suggest that both tree types were not physiologically active under extreme drought conditions. Alternative processes causing deuterium fractionation, however, could not be ruled out. Tree height and recent (last 15-25 years) growth rates in all species studied were lower in D than in ND trees by 22 and 44%, respectively. Lastly, there was not a consistent pattern of NSC sapwood concentration; in Q. pubescens, it was higher in ND trees while in Q. frainetto, the D trees were the ones exhibiting the higher NSC concentration. We conclude that the vulnerability to drought among conspecific Mediterranean oaks depends on the differential access to deep soil water pools, which may be related to differences in rooting depth, tree size and growth rate.This research was financially supported by the project OT4CLIMA (Italian Ministry of Education, University and Research (MIUR), Project N. ARS01_00405) ‘Advanced EO Technologies for studying climate change impacts on the environment’ and by the project ‘Alarm of forest mortality in Southern Italy’ (Gorgoglione Administration, Basilicata Region, Italy). M.C. was supported by the PhD program from the University of Basilicata (Italy). J.J.C. acknowledges funding by the project CGL2015-69186-C2-1-R project (Spanish Ministry of Economy). We acknowledge the E-OBS dataset from the EU-FP6 project UERRA (http://www.uerra.eu) and the data providers in the ECA&D project (https://www.ecad.eu)

Stomatal conductance and leaf water potential responses to hydraulic conductance variation in Pinus pinaster seedlings

In this study, tree hydraulic conductance (K tree) was experimentally manipulated to study effects on short-term regulation of stomatal conductance (g s), net photosynthesis (A) and bulk leaf water potential (Ψleaf) in well watered 5–6 years old and 1.2 m tall maritime pine seedlings (Pinus pinaster Ait.). K tree was decreased by notching the stem and increased by progressively excising the root system and stem. Gas exchange was measured in a chamber at constant irradiance, vapour pressure deficit, leaf temperature and ambient CO2 concentration. As expected, we found a strong and positive relationship between g s and K tree (r = 0.92, P = 0.0001) and between A and K tree (r = 0.9, P = 0.0001). In contrast, however, we found that the response of Ψleaf to K tree depended on the direction of change in K tree: increases in K tree caused Ψleaf to decrease from around −1.0 to −0.6 MPa, but reductions in K tree were accompanied by homeostasis in Ψleaf (at −1 MPa). Both of these observations could be explained by an adaptative feedback loop between g s and Ψleaf, with Ψleaf prevented from declining below the cavitation threshold by stomatal closure. Our results are consistent with the hypothesis that the observed stomatal responses were mediated by leaf water status, but they also suggest that the stomatal sensitivity to water status increased dramatically as Ψleaf approached −1 MPa

Drought-induced oak decline in the western mediterranean region: An overview on current evidences, mechanisms and management options to improve forest resilience

Increased forest vulnerability is being reflected as more widespread and severe drought-induced decline episodes. In particular, the Mediterranean area is revealing a high susceptibility to phenomena of loss in tree vitality across species. Within tree species, oaks (Quercus spp.) are experiencing extensive decline in many countries. However, in the wake of the so-called âoak decline phenomenonâ, the attention on these species has generally been limited. In this paper, we review the current available literature on oakdecline cases reported within the Mediterranean Basin, with particular remark for those occurred in Italy and Spain. More specifically our main aims were to: (i) provide an update on the patterns and mechanisms of decline by focusing on tree-ring and wood-anatomical variables; (ii) provide some hints for improving the resistance and resilience of oak stands experiencing decline. Our review reveals that drought is reported as the main driver triggering oak decline within the Mediterranean Basin, although other causes (i.e., increasing temperature, pathogens attack or excessive stand density) could exacerbate decline. In most reported cases, drought induced a substantial reduction of growth and changes in some wood anatomical properties. Indeed, growth decline prior death is also indicated as an early-warning signal of impending death. In ring-porous oak species, declining trees were often characterized by a very low production of latewood and a decrease in lumen area of the widest earlywood vessels, suggesting a potential reduction of hydraulic conductivity. Moreover, hydraulic dysfunction is reported as the main cause of decline. Finally, regarding management actions that should be considered for improving the resilience of declining stands and preserve the species-specific stand composition, it could be useful to shorten the rotation period of coppice stands or promoting their gradual conversion towards high forests, and favoring more drought-resistant species should also be considered. In addition, regeneration prior to regeneration cuts should be improved by anticipating seed dispersal or by planting oak seedlings obtained from local germoplasm

Environmental Effects on Oxygen Isotope Enrichment of Leaf Water in Cotton Leaves

The oxygen isotope enrichment of bulk leaf water (Db) was measured in cotton (Gossypium hirsutum) leaves to test the Craig- Gordon and Farquhar-Gan models under different environmental conditions. Db increased with increasing leaf-to-air vapor pressure difference (VPd) as an overall result of the responses to the ratio of ambient to intercellular vapor pressures (ea/ei) and to stomatal conductance (gs). The oxygen isotope enrichment of lamina water relative to source water ðD1Þ; which increased with increasing VPd, was estimated by mass balance between less enriched water in primary veins and enriched water in the leaf. The Craig-Gordon model overestimated Db (and D1Þ; as expected. Such discrepancies increased with increase in transpiration rate (E), supporting the Farquhar-Gan model, which gave reasonable predictions of Db and D1 with an L of 7.9 mm, much less than the total radial effective length Lr of 43 mm. The fitted values of L for D1 of individual leaves showed little dependence on VPd and temperature, supporting the assumption that the Farquhar-Gan formulation is relevant and useful in describing leaf water isotopic enrichment

Exploring the Use of Sentinel-2 Data to Monitor Heterogeneous Effects of Contextual Drought and Heatwaves on Mediterranean Forests

The use of satellite data to detect forest areas impacted by extreme events, such as droughts, heatwaves, or fires is largely documented, however, the use of these data to identify the heterogeneity of the forests’ response to determine fine scale spatially irregular damage is less explored. This paper evaluates the health status of forests in southern Italy affected by adverse climate conditions during the hot and dry summer of 2017, using Sentinel-2 images (10m) and in situ data. Our analysis shows that the post-event—NDVI (Normalized Difference Vegetation Index) decrease, observed in five experimental sites, well accounts for the heterogeneity of the local response to the climate event evaluated in situ through the Mannerucci and the Raunkiaer methods. As a result, Sentinel-2 data can be effectively integrated with biological information from field surveys to introduce continuity in the estimation of climate change impacts even in very heterogeneous areas whose details could not be captured by lower resolution observations. This integration appears to be a successful strategy in the study of the relationships between the climate and forests from a dynamical perspective

Impact of different nitrogen emission sources on tree physiology as assessed by a triple stable isotope approach

The importance that nitrogen (N) deposition has in driving the carbon (C) sequestration of forests has recently been investigated using both experimental and modeling approaches. Whether increased N deposition has positive or negative effects on such ecosystems depends on the status of the N and the duration of the deposition. By combining d13C, d18O, d15N and dendrochronological approaches, we analyzed the impact of two different sources of NOx emissions on two tree species, namely: a broadleaved species (Quercus cerris) that was located close to an oil refinery in Southern Italy, and a coniferous species (Picea abies) located close to a freeway in Switzerland. Variations in the ci/ca ratio and the distinction between stomatal and photosynthetic responses to NOx emissions in trees were assessed using a conceptual model, which combines d13C and d18O. d15N in leaves, needles and tree rings was found to be a bioindicator of N input from anthropogenic emissions, especially at the oil refinery site.We observed that N fertilization had a stimulatory effect on tree growth near the oil refinery, while the opposite effect was found for trees at the freeway site. Changes in the ci/ca ratio were mostly related to variations in d13C at the freeway site and, thus, were driven by photosynthesis. At the oil refinery site they were mainly related to stomatal conductance, as assessed using d18O. This study demonstrates that a single method approach does not always provide a complete picture of which physiological traits are more affected by N emissions. The triple isotope approach combined with dendrochronological analyses proved to be a very promising tool for monitoring the ecophysiological responses of trees to long-term N deposition

The IASI Water Deficit Index to Monitor Vegetation Stress and Early Drying in Summer Heatwaves: An Application to Southern Italy

The boreal hemisphere has been experiencing increasing extreme hot and dry conditions over the past few decades, consistent with anthropogenic climate change. The continental extension of this phenomenon calls for tools and techniques capable of monitoring the global to regional scales. In this context, satellite data can satisfy the need for global coverage. The main objective we have addressed in the present paper is the capability of infrared satellite observations to monitor the vegetation stress due to increasing drought and heatwaves in summer. We have designed and implemented a new water deficit index (wdi) that exploits satellite observations in the infrared to retrieve humidity, air temperature, and surface temperature simultaneously. These three parameters are combined to provide the water deficit index. The index has been developed based on the Infrared Atmospheric Sounder Interferometer or IASI, which covers the infrared spectral range 645 to 2760 cm−1 with a sampling of 0.25 cm−1. The index has been used to study the 2017 heatwave, which hit continental Europe from May to October. In particular, we have examined southern Italy, where Mediterranean forests suffer from climate change. We have computed the index’s time series and show that it can be used to indicate the atmospheric background conditions associated with meteorological drought. We have also found a good agreement with soil moisture, which suggests that the persistence of an anomalously high water deficit index was an essential driver of the rapid development and evolution of the exceptionally severe 2017 droughts

Evaluating growth and intrinsic water-use efficiency in hardwood and conifer mixed plantations

Abstract Key message Juglans, Fraxinus, Quercus and Pinus species seem to better maximize the carbon–water ratio providing useful indications on species selection for forestry plantations in areas with increasing drought risk. Abstract Maximizing carbon sequestration for a given water budget is extremely important in the contest of climate change in the Mediterranean region, which is characterized by increasing temperatures and rising water stress. This issue is fundamental for plantation stands, where limited water availability during the growing season reduces CO2 assimilation and, consequently, tree growth. In this study, the main objective was to investigate the performances in terms of carbon–water balance of conifer (Pinus halepensis and Cupressus sempervirens) and hardwood (Quercus robur, Juglans regia, Fraxinus excelsior and Populus spp.) mixed plantations. To this aim, we used carbon isotope signatures to evaluate the intrinsic water-use efficiency (iWUE) and the species-specific relationship between basal area increments (BAI) and iWUE. At the species level, the highest iWUE values corresponded to the lowest carbon accumulation in terms of BAI, for water-saving species such as Cupressus. Conversely, Populus had the lowest iWUE and the highest BAI accumulation. Juglans, Fraxinus, and Pinus showed the most balanced ratio between BAI and iWUE. Overall, no clear correlation of iWUE and BAI was evident within all species, except for Populus and Cupressus. Considering projected aridification and increased temperatures that will negatively impact the growth, our data suggest that Pinus, for conifers, and Quercus, Juglans, Fraxinus for hardwood species should be preferred when choosing species for forestry plantation, as they performed better in terms of BAI and iWUE ratio

Climate, drought and hydrology drive narrow-leaved ash growth dynamics in southern European riparian forests

Abstract Mediterranean riparian forests are among the most threatened ecosystems in Europe. These ecosystems are exposed to land-use changes threatening their reduced habitat and by global warming, which is already triggering aridification processes. To assess the impact of these major threats, we studied the radial-growth responses to climate and drought in the narrow-leaved ash (Fraxinus angustifolia). This riparian tree species presents a relatively large ecological spectrum in its habitat preference in the Mediterranean Basin. We studied five sites arranged across a wide geographical range from Iberia to Italy, subjected to contrasting climatic conditions and located in hydrographic basins with different sizes and water regimes. We found diverse growth responses to climate and drought across the Mediterranean distribution range of the narrow-leaved ash at the individual and site levels. The growth of this species increased in response to wet and cool conditions in the prior winter and spring. The response to summer conditions was only observed in the coldest and wettest site (Ticino). Growth responded negatively to 2–14 month droughts that occurred from previous winter up to summer, particularly in the warmest-driest sites. Growth responses to drought peaked in the warmest-driest sites in terms of climate water balance (Odelouca, Donana), but not in the driest sites in terms of annual precipitation (Tudela, Zaragoza). Hydrological conditions also affected the narrow-leaved ash with high discharges in the prior winter and early spring enhancing wood production. Considering projected aridification and increased hydrological alteration, implying limited water supply in the Mediterranean region, climate warming will negatively impact productivity of narrow-leaved ash riparian forests. Further research should combine analyses of growth responses to climate and hydrology from tree to basin scales to disentangle their relative roles as drivers of productivity under different scenarios of climate and hydrological changes, in order to aid adaptive management of these key ecosystems

Drought and Phytophthora Are Associated With the Decline of Oak Species in Southern Italy

Forest decline induced by climate change is a global phenomenon that affects many tree species, mainly in drought-prone areas as the Mediterranean region. In southern Italy, several oak species have shown decline symptoms and elevated mortality since the 2000s due to drought stress. However, it remains to be answered whether decline occurred alone or whether a pathogen was also involved. To this aim, we compared two coexisting oak species in a forest located in southern Italy which are assumed to be less (Quercus cerris) and more tolerant to drought (Quercus pubescens). We sampled fifteen couples of neighboring declining (D) and non-declining (ND) trees of both species. Wood cores were taken from all trees to perform dendrochronological analyses to detect the decline onset and link it to potential climatic drivers. Carbon isotope ratios (d13C) were analyzed in wood of the two vigor classes to compare their water-use efficiency. Phytophthora presence was also assessed in soil samples from ten D-ND couples of trees per species. The oak species most affected by drought-induced decline in terms of leaf shedding and mortality was Q. cerris, i.e., the least tolerant to drought. In both species, the D trees showed a reduced growth rate compared with ND trees from 2000 onward when drought and warming intensified. Q. pubescens showed higher growth sensitivity to precipitation, temperature and drought than Q. cerris. This sensitivity to climate was magnified in D trees whose growth decreased in response to warm and dry conditions during the prior winter and the late summer. The Q. pubescens D trees were more efficient in their water use than ND trees before the growth divergence between D and ND trees amplified. In the studied area, Phytophthora quercina was isolated from 40% of the sampled trees, and tended to be more frequent amongst ND than amongst D trees. Our data suggests that droughts and warm summer conditions triggered oak decline. The high prevalence of P. quercina in the studied area warrants further study as a potential predisposing factor